Various electrical current and electrical power sensors, based on direct or indirect electrical measurement methods, are readily available on the market. An old method still in use today consists of measuring the voltage drop across a resistor placed in the current path. There also exist more sophisticated techniques based on the Hall effects or magnetoresistive effects. Although these electrical measurement methods have been widely used, there are still today specific applications where they fail to provide correct measurements.
For example, these types of sensors often cannot be used in presence of strong electromagnetic fields or in high voltage environments. More particularly, these types of sensors are generally not well adapted for the assessment of electro-explosive devices (EED). Bridge-wire or hot-wire EED are used to initiate explosive components in ordnance and car airbag, for example. Current from an external source is applied to the resistive bridge-wire to raise its temperature, which in turn heats up the explosive components to its ignition point. EEDs are susceptible to initiation by electromagnetic fields as a result of current induced in the EED circuit. This can happen, for example, when the EED is located closed to high power communication system and radar transmitter. For safety reasons, the compatibility assessment of these devices is of great importance.
Several methods have been developed to measure the temperature rise in the EED bridge-wire due to the presence of strong electromagnetic (EM) fields. The most common one has been to use a thermocouple located in close proximity to the bridge-wire. This method has disadvantages: the response of the thermocouple sensor may be altered by the presence of EM fields and alternatively, the electrical leads of this sensor may alter the EM field under which the EED is being tested. Other methods have been described in the prior art to circumvent these problems. Most of these methods are based on the use of an optical sensor. U.S. Pat. No. 5,145,257 to Bryant et al. discloses the use of a temperature sensor comprising a probe, infrared fiber, super-cooled detector and associated electronics. Temperature rise of the bridge wire is measured by detecting the infrared light radiation emitted by the bridge-wire. This method suffers from poor sensitivity, authors claiming a sensitivity of 20 dB below the no fire threshold (NFT) point while acceptable sensitivity required for ordnance components is in the range of 35 dB.
Another proposed method consists of using a phosphorescent material. This material is located nearby or in contact with the bridge-wire. This material is exited by an external light source and its fluorescent decay time, which depends of temperature, is measured with an optical detector. One of the drawbacks of this technique is its slow response time.
A further method also described in the prior art uses a layer of transparent material whose refractive index varies with temperature. This material is sandwiched between two or more layers of reflective material. This sensor assembly forms a Fabry-Perot interferometer and is placed at the tip of an optical fiber. Light is sent to the sensor assembly which is located nearby or in contact with the bridge-wire. The spectrum of the reflected light is modulated according to refractive index variation due to temperature increase in the bridge wire. This spectrum modulation is measured by interferometric or spectrophotometric methods. This method comes with drawbacks as it is based on an interferometric principle. Among others, the method suffers form great sensitivity to mechanical vibration and optical fiber movement due to induced modal noise in the multimode optical fiber.
U.S. Pat. No. 5,021,731 Saaski et al. discloses the use of a thermo-optical sensor for measuring the current flowing between a pair of conductors. The sensor includes an optical sensing element having a resistive or semiconductive electrical property and an optical property that is function of temperature. The disadvantage of this method is that the electrical property of the sensing element may be affected by the presence of strong EM fields. These fields may also induced current in the electrical conductors that connect to the sensing element therefore giving false measurements.
U.S. Pat. No. 4,790,669 to Christensen discloses a temperature optical sensor. The optical sensor uses a semiconductor sensor having an optical absorption edge which is characteristic of the semiconductor material and is variable as a function of the temperature. Various optical arrangements are provided.